Metal materials and alloy materials have been widely utilized in various fields, for example as cladding materials, due to excellent mechanical properties and grave and massive appearance thereof. Especially, magnesium or magnesium alloy materials have the lightest weight among practically-used metal materials and exhibit a good machinability, a high ratio of strength to density and a good castability for die-cast, so that many studies have been made to apply the magnesium or magnesium alloy materials to various fields such as casings, structural members or various parts of computers, audio equipment, communication equipment, air planes, automobiles or the like.
However, the magnesium or magnesium alloy materials is deteriorated in anti-corrosion property and therefore readily anodized in an atmosphere so that a thin oxide film is readily produced on a surface thereof. Especially, when such magnesium or magnesium alloy materials are subjected to precision machining, there occurs a remarkable difference in an anti-corrosion property between respective surface regions thereof. Not only the magnesium or magnesium alloy materials but also other general metal materials poses this problem.
Therefore, in order to enhance an anti-corrosion property, an impact resistance, a film-adhesion property and the like, the magnesium or magnesium alloy materials have been conventionally subjected to an anodization treatment or other chemical treatments in which heavy metal salts such as chromates (containing hexavalent (VI) chromium), manganates or permanganates, or fluorides are used.
However, when the magnesium or magnesium alloy materials undergo the anodization treatment or other chemical treatments, there arises a serious problem that an inherent metallic appearance thereof is lost thereby.
For example, in the case where the magnesium or magnesium alloy materials are subjected to the anodization treatment or the other chemical treatments using heavy metal salts, an effluent resulting from each treatment is severely contaminated by the heavy metal salts. This is unfavorable from the standpoint of environmental protection.
Further, when the anodization treatment is adopted, there arise the following inconveniences.
That is, the anodic oxide film formed by the anodization treatment have a surface roughness three to ten times those of untreated magnesium or magnesium alloy materials. For this reason, it is extremely difficult for the anodized magnesium or magnesium alloy materials to attain aimed dimension after subjected to machining. Therefore, the machined magnesium or magnesium alloy materials are generally subjected to an abrasion process. However, since such an anodic oxide film is hard but brittle, there is a likelihood that falling-off of the anodic oxide film is caused at uneven sites thereof upon abrasion.
The anodic oxide film is provided with a huge number of pores each having a complicated shape and a diameter on the order of 3 to 10 .mu.m. Such powders generated during the abrasion process enters into or adhered to the pores or the uneven sites of the anodic oxide film. When the powder is fallen off during the use, it functions as an abrasive so that the anodic oxide film is apt to undergo self-destruction.
Since the anodic oxide film has a large surface roughness as described above, there arises a further inconvenience that the thickness thereof is difficult to control.